Physics degree, Caltech, then over thirty years designing airborne radars, including five years designing satellite radar & satellite communications networks (i.e., X- and Ku bands). At those wavelengths, signals transmitted by your phone (a critical part of all this) are not going more than 10-20-ish centimeters deep in the best conditions. So, from the perspective of mountains, RF signals are “line of sight” in that mountains are opaque to RF signals.

This is different from wireless signals getting through house or apartment walls, because they’re basically made of air. (And signals are aided by reflections from a myriad other surfaces around typical homes).